---
name: "dsprfs"
section: "3lapack"
platform: "linux"
package: "liblapack-doc"
version: "3.2.1-8"
release: "Debian 6.0 (Squeeze)"
platforms: ["linux"]
url: "https://new.carta.tech/dsprfs/3lapack"
---

# dsprfs(3lapack)

improves the computed solution to a system of linear equations when the coefficient matrix is symmetric indefinite and packed, and provides error bounds and backward error estimates for the solution

## NAME

DSPRFS - improves the computed solution to a system of linear equations when the coefficient matrix is symmetric indefinite and packed, and provides error bounds and backward error estimates for the solution

## SYNOPSIS

- SUBROUTINE DSPRFS(

  UPLO, N, NRHS, AP, AFP, IPIV, B, LDB, X, LDX, FERR, BERR, WORK, IWORK, INFO )

- CHARACTER

  UPLO

- INTEGER

  INFO, LDB, LDX, N, NRHS

- INTEGER

  IPIV( \* ), IWORK( \* )

- DOUBLE

  PRECISION AFP( \* ), AP( \* ), B( LDB, \* ), BERR( \* ), FERR( \* ), WORK( \* ), X( LDX, \* )

## PURPOSE

DSPRFS improves the computed solution to a system of linear equations when the coefficient matrix is symmetric indefinite and packed, and provides error bounds and backward error estimates for the solution.

## ARGUMENTS

- UPLO (input) CHARACTER\*1

  \= 'U':  Upper triangle of A is stored;\
   \= 'L':  Lower triangle of A is stored.

- N (input) INTEGER

  The order of the matrix A.  N \>= 0.

- NRHS (input) INTEGER

  The number of right hand sides, i.e., the number of columns of the matrices B and X.  NRHS \>= 0.

- AP (input) DOUBLE PRECISION array, dimension (N\*(N+1)/2)

  The upper or lower triangle of the symmetric matrix A, packed columnwise in a linear array.  The j-th column of A is stored in the array AP as follows: if UPLO = 'U', AP(i + (j-1)\*j/2) = A(i,j) for 1\<=i\<=j; if UPLO = 'L', AP(i + (j-1)\*(2\*n-j)/2) = A(i,j) for j\<=i\<=n.

- AFP (input) DOUBLE PRECISION array, dimension (N\*(N+1)/2)

  The factored form of the matrix A.  AFP contains the block diagonal matrix D and the multipliers used to obtain the factor U or L from the factorization A = U\*D\*U\*\*T or A = L\*D\*L\*\*T as computed by DSPTRF, stored as a packed triangular matrix.

- IPIV (input) INTEGER array, dimension (N)

  Details of the interchanges and the block structure of D as determined by DSPTRF.

- B (input) DOUBLE PRECISION array, dimension (LDB,NRHS)

  The right hand side matrix B.

- LDB (input) INTEGER

  The leading dimension of the array B.  LDB \>= max(1,N).

- X (input/output) DOUBLE PRECISION array, dimension (LDX,NRHS)

  On entry, the solution matrix X, as computed by DSPTRS. On exit, the improved solution matrix X.

- LDX (input) INTEGER

  The leading dimension of the array X.  LDX \>= max(1,N).

- FERR (output) DOUBLE PRECISION array, dimension (NRHS)

  The estimated forward error bound for each solution vector X(j) (the j-th column of the solution matrix X). If XTRUE is the true solution corresponding to X(j), FERR(j) is an estimated upper bound for the magnitude of the largest element in (X(j) - XTRUE) divided by the magnitude of the largest element in X(j).  The estimate is as reliable as the estimate for RCOND, and is almost always a slight overestimate of the true error.

- BERR (output) DOUBLE PRECISION array, dimension (NRHS)

  The componentwise relative backward error of each solution vector X(j) (i.e., the smallest relative change in any element of A or B that makes X(j) an exact solution).

- WORK (workspace) DOUBLE PRECISION array, dimension (3\*N)

  - IWORK (workspace) INTEGER array, dimension (N)

  - INFO (output) INTEGER

  \= 0:  successful exit\
   \< 0:  if INFO = -i, the i-th argument had an illegal value

## PARAMETERS

ITMAX is the maximum number of steps of iterative refinement.